This invention relates to a method of removing organic and inorganic sulfur compounds from coal and other carbonaceous combustible materials.
The recent energy crisis has increased the consumption of coal in the United States. However, there are many problems which need to be solved concerning the use of coal, the most important of which is environmental pollution. Most coal found in the United States contains from 0.5 to 10 weight percent sulfur which when burned is emitted as sulfur dioxide causing serious pollution problems in the atmosphere. Moisture in the atmosphere combines with the sulfur dioxide to produce acid rain, the consequences of which are far reaching and deleterious.
Because of the adverse impact of large quantities of sulfur from burning coal, it is necessary to reduce substantially the amount of sulfur which is released to the atmosphere. Only a small fraction of the available coal can be burned directly without violating current pollution control regulations. Thus, methods are being developed either to decrease the amount of sulfur in the coal before it is combusted or to remove sulfur from flue gas. However, flue gas desulfurization is expensive because of the high cost of the capital equipment and the cost of maintaining that equipment. Precombustion processes include conventional physical cleaning such as by comminution, chemical treatment, magnetic separation and coal conversion such as gasification and liquification.
Most physical cleaning methods separate mineral impurities from coal, utilizing differences in density of coal and mineral matter. This method only removes coarse mineral particles which are easily released, while leaving the finer particles in the coal. Coal gasification and liquification are not yet fully developed and are expensive. Magnetic separation can remove only liberated particles but does remove some ash forming minerals in addition to some of the sulfur. Chemical cleaning methods, which remove both organic and inorganic sulfur, are more effective than physical cleaning methods and are generally more economical than gasification and/or liquification.
Sulfur in coal may be classified into two general types, organic and inorganic. Organic sulfur is chemically bounded to the coal hydrocarbon matrix can only be removed by chemical means. Inorganic sulfur is present in coal with pyrite (FeS.sub.2) and in small amounts as a sulfate (generally, calcium or ferrous sulfate). Pyritic sulfur may range from about 0.5 to abut 10 percent by weight with individual particles of pyrite ranging from a few microns to a few inches in diameter. Large, liberated pyrite particles can be removed by hand or by various mechanical cleaning methods, but the particles that are finely distributed in the coal matrix need first to be liberated by fine grinding. Grinding and overgrinding produces a large percentage of fine material which is expensive not only because grinding to fine materials is costly but also the finer the coal, the more difficult in processing thereafter.
A number of different processes have been developed for the removal of organic and inorganic sulfur from coal and other burnable carbonaceous materials for reducing the ash content thereof. One method is known as the Gravimelt Process. The method is based on treating one part of finely powdered coal with approximately ten parts of a fused alkali such as sodium hydroxide, potassium hydroxide or mixtures thereof at 300.degree. C. to 400.degree. C. for 20-70 minutes. After removal of the coal floating on top of the melt and washing extensively with water to remove residual alkali and reaction products, substantial reductions in the sulfur content can be achieved. Subsequent washing of the treated coal with dilute sulfuric acid removes much of the mineral matter and neutralizes any remaining alkali, leaving a product that is relatively low in sulfur and low in ash content. Approximately ten percent of the original sulfur is present and approximately two percent ash is present by weight of the final product.
In another process for removing sulfur, a slurry of finely divided coal is in a solvent of methylchloroform, carbon tetrachloride or tetrachloroethylene and 30-70 weight percent water is prepared. Gaseous chlorine is bubbled through the slurry at 60.degree.-130.degree. C. and from 0-60 psig for about 45-90 minutes to oxidize the coal. The process will remove about 60 percent of the total sulfur in the coal removing about 50 percent of the organic sulfur and about 70 percent of the pyritic sulfur. While the process is reasonably effective, it is not as effective as the Gravimelt Process leaving about 50 percent of the organic sulfur in the coal. In addition, substantial amounts of residual chlorine remain in the coal which can produce highly corrosive combustion products upon burning.
The patent to Aida et al. U.S. Pat. No. 4,497,636 issued Feb. 5, 1985 attempts to combine the Gravimelt technology and the chlorinating technology in which chlorine gas is used as an oxidant and thereafter the carbonaceous material separated from the liquid chlorine is contacted with molten caustic, the operating temperatures used being in the range of from about 250.degree.-400.degree. C. with about 325.degree. C. being preferred. The results reported are about 90 percent of the total sulfur present in the coal being removed. Lower concentrations of alkali have been used to remove sulfur as reported in the Reggel et al. U.S. Pat. No. 3,993,455 and alkali has been used in super critical fluid conditions supposedly to release sulfur as reported in the Narain et al. U.S. Pat. No. 4,775,387. The three patents referred to, these being the Reggel et al. patent, the Aida et al. patent and the Narain et al. patent are all assigned to the assignee of the present invention and illustrate a portion of the resources of the government devoted to finding better and cleaner methods for burning coal.